Add test coverage for epic metadata and confidentiality

What does this MR do and why?

While exploring the idea of mutation testing in https://gitlab.com/gitlab-com/gl-security/appsec/appsec-reviews/-/issues/66, it was discovered that some permissions in epics weren't tested. This MR adds test coverage for those permissions.

Screenshots or screen recordings

Only specs were changed.

How to set up and validate locally

Only specs were changed.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.